The present invention relates to a monitoring device and a method for the independent monitoring of the presence of a first flame and of a second flame in such a fuel combustion device.
Fuel combustion devices are used in, among other things, heating and/or hot water engineering and in industrial thermoprocessing equipment employed, for example, in the smelting of metals or the firing of ceramics. Fuel combustion devices which are used in heating and/or hot water engineering and are contained in boilers or continuous flow water heaters do not normally only operate with a main flame, for actual heat generation, but also have a second, so-called pilot flame, which is also called an ignition flame. In this document, fuel burning devices which are also described as burner automats, fuel burners or simply burners can, for example, be gas burners or oil burners.
In many types of burner, the pilot flame does not go out even when no heat output is required. The purpose of the pilot flame is thus only to assure rapid ignition of the main flame when a fuel valve assigned to the main flame is opened. In other types of burner the pilot flame is extinguished in the interim and is only reignited shortly before the main flame ignites, for example by an electric ignition. When the main flame is activated, the burning pilot flame ensures that the main flame ignition process can take place in a controlled and ordered manner.
For reasons of operational reliability, relevant standards state that it must be possible to detect the presence of the main and/or the pilot flame independently of one another.
The use of a UV detector for monitoring the presence of both a pilot flame and a main flame is known from technical research documentation such as “Flammenüberwachung an Öl-und Gasbrennern”, Siemens Building Technologies, CC1Z7302de, HVAC Products, 16.02.2005. A UV detector of this kind, which is also called a UV flame sensor, is formed of a series circuit made up of ohmic resistance, a UV cell and a diode.
The UV cell of the UV detector has a glass flask made of UV-permeable quartz glass and filled with noble gas. In the glass flask are two electrodes. If voltage is applied between the two electrodes and this voltage is increased, then when a critical voltage is reached, glow discharge (ignition) takes place. Electrons are emitted from the negative electrode, are accelerated toward the positive electrode and ionize the noble gas. This results in a flow of current through the UV cell. The UV cells used for flame monitoring typically exhibit this behavior of self-ignition only at voltages of more than 700 V. UV cells behave differently if they are irradiated by the flame being monitored with UV light of which the wavelength is approx. 190 to 260 nm: in such cases, occurrence of the ignition effect depends on the intensity of the UV radiation at effective voltages as low as approx. 200 V.
The diode of the UV detector assures half-wave rectification, so that if the UV detector is operating with an alternating voltage when a flame is present a pulsed direct voltage is generated. In the event of a short circuit in the connection cables of the UV detector it, and thus also the diode, is bypassed, with the result that when the UV detector is operating with an alternating voltage, alternating voltage is generated instead of the pulsed direct voltage, including at the input of an amplifier circuit downstream from the UV detector. In this way, the presence of a flame is indicated by a pulsed direct voltage and a cable short circuit is indicated by an alternating voltage.
Monitoring the main flame and the pilot flame independently of one another using a UV detector in each case has the disadvantage that (a) a connection cable (with two connection wires) and (b) an amplifier circuit are needed for each of the two UV detectors. Monitoring the main flame and the pilot flame using a UV detector in each case is thus relatively apparatus-intensive.